1. Field of the Invention
The present invention relates to a bonding method and apparatus and more particularly to a method and apparatus for correcting the offset amount between the optical axis of a position-detecting camera, which detects the bonding position, and the axial center of the tool.
2. Prior Art
A general structure of a wire bonding apparatus will be first presented below.
A position-detecting camera which is used to detect the positions of bonding points on semiconductor devices, and a bonding arm which has a bonding tool that performs bonding attached to one end, are disposed on a bonding head which is mounted on an XY table. These parts are provided on the bonding head so that the optical axis of the position-detecting camera and the axial center of the tool are separated (or are not aligned) by a predetermined distance so that the tool and bonding arm do not interfere with the field of vision of the position-detecting camera when the position-detecting camera detects the position of a bonding point. Generally, this distance between the optical axis of the position-detecting camera and the axial center of the tool is called xe2x80x9coffsetxe2x80x9d.
Since the position-detecting camera is used to determine a reference point for ascertaining the position to which the tip end of the tool is to be moved, it is necessary that the distance by which the position-detecting camera is offset from the tool be accurately set. Examples of conventional offset setting methods are disclosed in, for example, Japanese Patent Application Laid-Open (Kokai) No. S59-69939 (hereafter referred to as xe2x80x9cConventional Example 1xe2x80x9d), Japanese Patent Application Laid-Open (Kokai) No. H6-224248 (hereafter referred to as xe2x80x9cConventional Example 2xe2x80x9d), and Japanese Patent No. 2568125 (hereafter referred to as xe2x80x9cConventional Example 3xe2x80x9d).
In Conventional Example 1, an image of the pressure mark of the tool is photoelectrically converted by a position-detecting camera (photoelectric transducer), and the center of the pressure mark of the tool is determined by processing the resulting image signal; then, the offset is determined and set in accordance with this information.
In Conventional Example 2, one pad on the semiconductor chip is imaged by a position-detecting camera, and the center coordinates of the pad are determined by processing this image data. Next, trial bonding of the ball formed on the tip end of the wire is performed by the tool in the vicinity of the pad. Then, the XY table is driven, and the trial-bonded ball is imaged by the position-detecting camera, after which the center coordinates of the ball are determined by processing this image data. The offset is set in accordance with the center coordinates of the pad and the center coordinates of the ball.
The bonding stage contains a heater and is heated to a high temperature by this heater; as a result, the position-detecting camera holder and bonding arm are caused to undergo thermal expansion by the resulting radiant heat. Consequently, the amount of offset changes according to the difference between the amount of elongation of the position-detecting camera holder and the amount of elongation of the bonding arm caused by the thermal expansion, so that a positional discrepancy is generated in the bonding position. Correction of the amount of offset resulting from this thermal effect can also be accomplished using the methods of the Conventional Examples 1 and 2.
In Conventional Example 3, a tool edge detector consisting of an optical fiber detector that detects the edge of the tool is provided, and the position of the position-detecting camera does not change. Instead, discrepancies in the amount of offset generated as a result of changes in the tool position caused by thermal effects are corrected. In other words, only discrepancies in the position of the tool caused by thermal effects are detected.
It is assumed that the offset is accurately set beforehand, and only the correction of discrepancies in the amount of offset caused by thermal effects is dealt with. This is disclosed in, for example, Japanese Patent Application Laid-Open (Kokai) Nos. H1-161727 and H4-343235.
In Conventional Examples 1 and 2, bonding is performed with the approximate offset adjusted, and the amount of offset is corrected and set according to the amount of deviation of the pressure mark of the tool or the position of the ball produced by this bonding. However, this method involves the problems as described below.
First of all, in Conventional Example 1, the amount of offset is corrected after bonding is performed in an unsatisfactory position. In other words, the next bonding position is corrected after a defective product is manufactured. In Conventional Example 2, in order to avoid the manufacture of such a defective product, trial bonding is performed in a position which has no effect on the actual bonding position of the lead of a lead frame of a lead frame. However, depending on the lead involved, there may be cases in which the space in which bonding can be performed is so narrow that no positional shift is allowed, so that there is an aversion to shifted bonding for the purpose of correction; accordingly, the method cannot be applied to such semiconductor devices. Furthermore, trial bonding and subsequent correction work result in a waste of production time, so that productivity is hindered.
Secondly, in regard to the magnification of the position-detecting camera used to detect positions in the bonding apparatus, a large visual field is required to some extent; accordingly, the magnification is low, and the detection of positions on the order of xcexcm is difficult. In particular, unlike patterns that are created exclusively for use in correcting positions, actual tool pressure marks or balls show individual differences in their image, so that the detection of accurate positions is difficult. Accordingly, it is described in Conventional Example 1 that a zoom lens is used; however, since an inner lens is moved in a zoom lens, the image position inevitably changes, thus causing the offset to change. Consequently, the use of a zoom lens is not realistic in a bonding apparatus, and detection is therefore performed xe2x80x9cas isxe2x80x9d at a low magnification, and the precision of positional detection is poor.
Third, since images of the tool pressure mark or ball at the time of bonding show individual differences, accurate positions cannot be determined.
In Conventional Example 3, no detection of the tool pressure mark or ball position is made at the time of bonding unlike Conventional Examples 1 and 2; accordingly, the problems arising in Conventional Examples 1 and 2 do not occur in Conventional Example 3. However, since only the fluctuation of the tool caused by thermal effects is detected in Conventional Example 3, the following problem arises:
Since the position-detecting camera is installed on the tool side and detects the position of the bonding point, this camera is susceptible to the effects of radiant heat arising from the high temperature of the bonding stage. As a result, the position-detecting camera holder undergoes thermal expansion, and a fluctuation in the position of the position-detecting camera cannot be avoided. Especially in cases where it is attempted to improve the bonding precision to sub-pm units as is required in a bonding apparatus, this cannot be satisfactorily achieved by detection of the positional fluctuation of the tool alone.
The object of the present invention is to provide a bonding method and apparatus which allows high-precision setting and correction of the offset without hindering productivity.
The method of the present invention which solves the prior art problems is used for a bonding apparatus in which a position-detecting camera that detects the position of a part to be bonded and a tool that performs bonding are offset, and in the method of the present invention:
the position-detecting camera is moved to a point above a reference marking provided on a reference member, and the positional relationship between the reference marking and the optical axis of the position-detecting camera is measured by the position-detecting camera;
the tool is moved to a point above the reference marking in accordance with an amount of offset stored in memory beforehand,
the positional relationship between the reference marking and the tool is measured by an offset-correcting camera, and
a desired accurate amount of offset is determined by correcting the amount of offset stored in memory beforehand on the basis of the measurement result.
The apparatus of the present invention which solves the prior art problems includes a position-detecting camera that detects the position of a part to be bonded and a tool that performs bonding that are offset, and further includes:
a reference member on which a reference marking is disposed,
an offset-correcting camera which detects the reference marking, and
a calculation and control device which determines an accurate amount of offset by correcting an amount of offset stored in memory beforehand on the basis of measurement result that consists of:
a measured value obtained when the position-detecting camera is moved to a point above the reference marking on the reference member, and the positional relationship between the reference marking and the optical axis of the position-detecting camera is measured by the position-detecting camera, and
another measured value obtained when the tool is moved to a point above the reference marking in accordance with the amount of offset stored in memory beforehand, and the positional relationship between the reference marking and the tool is measured by the offset-correcting camera.
In the method and apparatus described above, the reference member is a transparent member, and the offset-correcting camera is positioned beneath the reference member so as to face the reference member.
In addition, in the above method and apparatus, the offset-correcting camera is positioned to one side of the reference member so as to face the reference member.